Waveform encoding is a well-known encoding technique used for transmitting of video images. The object of waveform encoding is to approximate the waveform of the original input signal as well as possible at the available bitrate. The quality of the reconstructed signal generally decreases with a decreasing bitrate. Waveform encoders often apply the technique of transform coding. The widely accepted MPEG standard, for example, applies a Discrete Cosine Transform (DCT) to pixel blocks, and quantizes and variable-length codes the coefficients obtained from said transform. Other transforms are currently being investigated. For example, the overlapped transform, e.g. the Lapped Orthogonal Transform (LOT), promises to be suitable for encoding and compressing medical X-ray images. Even at very high compression ratios, the relevant clinical information seems to be present in the compressed images.
Model-based coding is a relatively new concept of image data compression. In model-based coding, the original input signal is first modeled, and quantized versions of the model parameters are transmitted to the decoder. Instead of producing a replica of the original waveform, the decoder generates a signal which looks very similar to the original signal, but may have a completely different waveform. A very high compression ratio can be achieved, with model-based coding.
Known hybrid waveform and model-based encoders and decoders are disclosed in "Model-based/waveform hybrid coding for videotelephone images", International Conference on Acoustics, Speech and Signal Processing (ICASSP) 91; Vol.4; 1991; pp. 2741-2744. The known arrangement applies model-based coding to head-and-shoulder images, and waveform encoding to the background image which cannot be modeled. The waveform coding is also used to cancel errors of the model-based analysis system.